A surface plasmon is an electromagnetic wave mode having properties of electric field localization and electric field enhancement. In recent years, the application research on the surface plasmon has been developed in diverse directions in nanotechnological and biotechnological fields. One field of the research is related to applications of a local surface plasmon. One of the applications is a plasmon polariton for transmitting light by use of metallic nanoparticles having a size in the nano order. For such applications, various methods for producing metallic nanoparticles have been studied. In order to make the signal intensity therefrom larger, studies have also been made for developing the structure thereof from a one-dimensional to a two-dimensional or three-dimensional structure.
In this field, the method for producing a metallic nanoparticle structure is most commonly electron beam lithography. For this technique, high-level CMOS technique and expensive device are essential. Additionally, it is basically difficult to produce a metallic nanoparticle three-dimensional structure.
Apart from the above-mentioned producing method using electron lithographic technique, studies have been actively made for producing metallic nanoparticles on the basis of chemical synthesis in order to produce metallic nanoparticles at lower cost. Examples of the studies include: a report showing that, in the Zsigmoddy method, which is a reduction method, the pH of a solution is rapidly changed in the reduction process so as to produce an Ag film of 10 to 20 nm thick uniformly on the surfaces of silica fine particles (80 to 180 nm), thereby producing Ag nano-shells; a report of putting silver particles into an aqueous gold chloride solution, and using the resultant as a reducing agent to produce Au shells on the surfaces of silver nanoparticles; and a report of using tin chloride as a reducing agent to produce silica spheres having surfaces on which silver nanoparticles are uniformly dispersed.
According to these chemical methods, metallic nanoparticles can be produced; however, the resultant samples are entirely in the form of a colloidal solution. When the particles are used as a device, it is necessary to make the particles into a solid form. However, in many researches, no study has been made on a process for dispersing nanoparticles in a transparent matrix without aggregating the nanoparticles.
As an intermediate technique between the above-mentioned inexpensive chemical synthesis methods and the highly accurate electron beam lithography, there is a technique of producing an Au-nanoparticle-dispersed SiO2 film by co-sputtering of a metal, such as Au, and SiO2, or some other method. According to this technique, an Au nanoparticle-dispersed composition can be three-dimensionally produced. In general, a large number of researches aim for nonlinear optical material using a local surface plasmon of Au or the like. A large amount of Au nanoparticles can be three-dimensionally incorporated into a transparent SiO2 film; however, according to a mere sputtering of Au, the crystallinity of the Au nanoparticles is bad and the particles are not spherical. Therefore, after the formation of the film, it is necessary in many cases to subject the film to heating treatment at 500° C. or higher (B. Zhang, H. Masumoto, Y. Someno and T. Goto, “Optical Properties of Au/SiO2 Nano-Composite Films Prepared by Induction-Coil-Coupled Plasma Sputtering”, Mater. Trans., 44[2], (2003), pp. 215-219). Additionally, the uniformity of the particle diameters is not easily obtained due to the thermal treatment. Moreover, it is unavoidable that the particle diameters are increased by the aggregation of the particles when the particles are heated. As a result, it is also difficult to produce fine metallic nanoparticles each having a diameter as fine as 20 nm or less.
As described above, producing methods using electron beam lithography have a problem that production costs increase. Conventional chemical synthesis methods do not provide any technique of making metallic nanoparticles into a solid form. Additionally, according to the co-sputtering method, it is difficult to make metallic nanoparticles into fine sizes and realize an uniform and highly dense distribution of the particles in a matrix.